Systems and methods for locating and guiding operative elements within interior body regions

ABSTRACT

Systems and methods for locating an operative element within an interior body space use a locating probe, which includes at least one transmitting element to transmit an electric waveform output within at least a portion of the space. The systems and methods also use a sensing element, which is adapted to be carried by the operative element to sense a local electric waveform within the space. A processing element coupled to the sensing element generates a processed output that locates the sensing element relative to the locating probe based, at least in part, upon a differential comparison of the waveform output and the sensed local waveform.

RELATED APPLICATION

This application is a continuation of application Ser. No. 08/745,795,filed Nov. 8, 1996 (now U.S. Pat. No. 3,941,251). Application Ser. No.08/745,795 is a continuation-in-part of application Ser. No. 08/679,156,filed Jul. 12, 1996 (now U.S. Pat. No. 5,722,402), which is acontinuation of application Ser. No. 08/320,301, filed Oct. 11, 1994(now abandoned), and is a continuation-in-part of application Ser. No.08/739,508, filed Oct. 28, 1996 (now U.S. Pat. No. 5,740,808). All ofthe above-related applications are fully and expressly incorporatedherein by reference.

FIELD OF THE INVENTION

The invention generally relates to systems and methods for guiding orlocating diagnostic or therapeutic elements in interior regions of thebody.

BACKGROUND OF THE INVENTION

Physicians make use of catheters today in medical procedures to gainaccess into interior regions of the body for diagnostic and therapeuticpurposes. It is important for the physician to be able to reliably andprecisely position in proximity to desired tissue locations. Forexample, the need for precise control over the catheter is especiallycritical during procedures that ablate myocardial tissue from within theheart. These procedures, called ablation therapy, are used to treatcardiac rhythm disturbances.

SUMMARY OF THE INVENTION

This invention has as its principal objective the realization of safeand efficacious systems and methods for remotely locating operativeelements at precise locations within the body.

The invention provides systems and methods for locating an operativeelement within an interior body space. The systems and methods use alocating probe, which includes at least one transmitting element totransmit an electric waveform output within at least a portion of thespace. The systems and methods also use a sensing element, which isadapted to be carried by the operative element to sense a local electricwaveform within the space. A processing element coupled to the sensingelement generates a processed output that locates the sensing elementrelative to the locating probe based, at least in part, upon adifferential comparison of the waveform output and the sensed localwaveform.

Other features and advantages of the inventions are set forth in thefollowing Description and Drawings, as well as in the appended Claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view, somewhat diagrammatic in form, of a systemto locate the position of an operative element within a space bygenerating a waveform energy field from a single locating probe;

FIG. 2 is a diagrammatic plan view of the system shown in FIG. 1,showing a representative position of the operative element relative towaveform phase iso-potential surfaces generated within the space;

FIG. 3 is a schematic view of an assembly of electrical components thatthe system shown in FIG. 1 can employ in carrying out its locatingfunctions;

FIG. 4 is a diagrammatic plan view of a system to locate the position ofan operative element within a space by generating a waveform energyfield from multiple locating probes, showing a representative positionof the operative element relative to the intersecting waveform phaseiso-potential surfaces generated within the space;

FIG. 5 is a perspective view, somewhat diagrammatic in form, of thesystem shown in FIG. 4;

FIG. 6 is a side view of an assemblage of multiple locating probes in acomposite structure, which is shown in an expanded condition ready foruse;

FIG. 7 is the composite locating probe structure shown in FIG. 6, exceptshown in a collapsed condition for deployment into a body region;

FIG. 8 is a diagrammatic plan view of a system to locate the position ofan operative element within a space using voltage differentialcomparisons between two locating probes;

FIG. 9 is a diagrammatic view of a three-dimensional system for locatingthe position and guiding movement of an operative element within aheart;

FIG. 10 is a diagrammatic view of a portion of the system shown in FIG.9, showing the inputs which set the system parameters to guide thecreation of a position-identifying output;

FIGS. 11 and 12 are plan views, somewhat diagrammatic in form, showingalternative implementations of a code to identify the geometry of alocating probe, which code serves as one of the inputs shown in FIG. 10;

FIG. 13 is a representative virtual image that the system shown in FIG.10 generates from the position-identifying output;

FIG. 14 is a diagrammatic view of a three-dimensional system forlocating the position and guiding movement of ablation elements within aheart;

FIG. 15 is a plan view of a representative continuous lesion pattern;

FIG. 16 is a plan view of an representative interrupted lesion pattern;

FIG. 17 is a perspective and somewhat diagrammatic view of a compositethree-dimensional basket structure of multiple locating probes usable inassociation with a central processing unit to derive alocation-indicating output using an iterative voltage distributionanalysis;

FIG. 18 is a flow chart showing the steps of an algorithm that thecentral processing unit shown in FIG. 17 can use to derive alocation-indicating output using an iterative voltage distributionanalysis;

FIG. 19 shows voltage distribution patterns, one actual and the otherestimated, which the algorithm shown in FIG. 18 iteratively matches inderiving a location-indicating output;

FIG. 20 is a diagrammatic plan view of a system to locate the positionof an operative element within a space by generating multiple frequencywaveforms from multiple locating probes;

FIG. 21 is a diagrammatic plan view of a system to locate the positionof an operative element within a space by generating multiple frequencywaveforms from a single locating probe; and

FIG. 22 is a perspective and somewhat diagrammatic view of a compositethree-dimensional basket structure of multiple locating probes usable inassociation an operative element that carries two electrodes fortransmitting different frequency waveforms for sensing by the locatingprobes.

The invention may be embodied in several forms without departing fromits spirit or essential characteristics. The scope-of the invention isdefined in the appended claims, rather than in the specific descriptionpreceding them. All embodiments that fall within the meaning and rangeof equivalency of the claims are therefore intended to be embraced bythe claims.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

I. Differential Waveform Analysis

A. Single Locating Probe

FIG. 1 shows a system 10, which locates the position of an operativeelement 12 within a space (designated S). The system 10 is well adaptedfor use inside body lumens, chambers or cavities for either diagnosticor therapeutic purposes. For this reason, the system 10 will bedescribed in the context of its use within a living body. The system 10particularly lends itself to catheter-based procedures, where access tothe interior body region is obtained, for example, through the vascularsystem or alimentary canal, without complex, invasive surgicalprocedures.

For example, the system 10 can be used during the diagnosis andtreatment of arrhythmia conditions within the heart, such as ventriculartachycardia or atrial fibrillation. The system 10 also can be usedduring the diagnosis or treatment of intravascular ailments, inassociation, for example, with angioplasty or atherectomy techniques.The system 10 also can be used during the diagnosis or treatment ofailments in the gastrointestinal tract, the prostrate, brain, gallbladder, uterus, and other regions of the body.

For deployment into an interior body space S, the operative element 12is carried in the illustrated embodiment at the distal end of a cathetertube 44. Nevertheless, the system 10 can also be used in associationwith systems and methods that are not necessarily catheter-based.

The operative element 12 can take different forms and can be used foreither therapeutic purposes, or diagnostic purposes, or both. Theoperative element 12 can comprise, for example, a device for imagingbody tissue, such as an ultrasound transducer or an array of ultrasoundtransducers, or an optic fiber element. Alternatively, the operativeelement 12 can comprise a device to deliver a drug or therapeuticmaterial to body tissue. Still alternatively, the operative element 12can comprise a device, e.g., an electrode, for sensing a physiologicalcharacteristic in tissue, such as electrical activity in heart tissue,or for transmitting energy to stimulate or ablate tissue.

The system 10 includes a locating probe 14, which, like the operativeelement 12, is carried at the distal end of a catheter tube 45 forintroduction into the body space S. In use, the locating probe 14establishes a localized field 20 comprising waveform energy in at leasta portion of the space S.

The system 10 provides a sensing element 16 on the operative element 12.When located within the energy field 20, the sensing element 16 acquireslocal characteristics of the energy field 20 surrounding it. The sensingelement 16 may be a component added to the operative element 12, or itmay comprise a component already on the operative element 12, but usedfor an additional purpose.

The system 10 further includes a central processing unit 18. The centralprocessing unit 18 receives as input the energy field characteristicacquired by the sensing element 16. The central processing unit 18derives a position-indicating output 42, which locates the position ofthe sensing element 16, and thus the operative element 12 itself,relative to the locating probe 14 within the space S.

In the illustrated embodiment, the central processing unit 18 includesan output display device 36 (e.g., a CRT, LED display, or a printer).The device 36 presents the position-indicating output 42 in a visualformat useful to the physician for remotely locating and guiding theoperative element 12 within the localized energy field 20 generated bythe locating probe 14. Further details for processing theposition-indicating output 42 for display will be described in greaterdetail later.

The system 10 includes an oscillator 22, which generates the waveformcomprising the energy field 20. In the illustrated embodiment, thecentral processing unit 18, which is coupled to the oscillator 22 by acontrol bus 24, conditions the oscillator 22 to generate an electricalalternating current (AC) waveform at a predetermined amplitude andfrequency.

For use within a living body space, the selected current amplitude ofthe oscillator output can vary between 0.1 mAmp to about 5 mAmp. Thefrequency selected can also vary from about 5 kHz to about 100 kHz. Whenthe space S is adjacent heart tissue, currents substantially above about5 mAmp and frequencies substantially below 5 kHz should be avoided, asthey pose the danger of inducing fibrillation. The maximum current is afunction of the frequency, as expressed in the following equation:

    I=ƒ×10

where I is current in μAmp, and f is frequency in kHz.

The shape of the waveform can also vary. In the illustrated andpreferred embodiment, the waveform is sinusoidal. However, square waveshapes or pulses can also be used, although harmonics may be encounteredif capacitive coupling is present. Furthermore, the waveform need not becontinuous. The oscillator 22 may generate pulsed waveforms.

The locating probe 14 carries at least one electrode 26(1) capable oftransmitting energy and at least one energy return electrode 28 capableof returning the energy to ground. These electrodes 26(1) and 28 areelectrically coupled to the oscillator 22 through an electronic switchunit 30. The locating probe 14 also carries at least one sensingelectrode (four such electrodes 26(2) to 26(5)are shown in FIG. 1),which are located between the transmitting electrode 26(1) and thereturn electrode 28. Preferably, the sensing electrode(s) 26(2) to 26(5)are also capable of becoming a transmitting electrode in place of theelectrode 26(1), to change the point of energy transmission, if desired.

For purposes of description, the illustrated embodiment shows the onereturn electrode 28 carried at the distal region 32 of the locatingprobe 14 and the other five electrodes 26(1) to 26(5) carried in aspaced-apart relationship along the probe axis 34, proximal of thereturn electrode 28, with the transmitting electrode 26(1) being themost proximal.

The number and placement of the electrode(s) 26 and return electrode(s)28 on the locating probe 14 can vary. Generally speaking, theposition-resolution capability of the system 10 improves with increasednumber of electrodes 26. Also generally speaking, theposition-resolution capability of the system 10 improves as the spacingbetween adjacent intermediate electrodes 26(2) to 26 (5) and the spacingbetween the transmitting electrode 26(1) and the return electrode 28decreases.

The geometry of the locating probe 14 itself can also vary. In theillustrated embodiment, the locating probe 14 takes the elongated,cylindrical form of a conventional diagnostic catheter, which is wellsuited for deployment in interior body regions.

In the illustrated embodiment, the central processing unit 18 is capableof connecting the waveform output of the oscillator 22 through theswitch unit 30 between the transmitting electrode 26(1) and the returnelectrode 28, which is coupled to isolated ground or patient ground 38.This creates an energy waveform field 20 emanating into at least aportion of the space S.

The central processing unit 18 is also capable of acquiring adifferential voltage between electrodes 26(1) to 26 (5) and the sensingelectrode 16 through another switch element 72 and a data acquisitionelement DAQ 68. The differential voltage measurements are taken alongiso-potential surfaces 40(1) to 40(5) in the energy waveform field 20.

FIG. 1 shows the iso-potential surfaces associated with electrodes26(1), 26(2), 26(3), 26(4), and 26(5) as, respectively, planes 40(1),40(2), 40(3), 40(4), and 40(5). FIG. 2 shows the energy field 20 and theiso-potential surfaces 40(1) to 40(5) in plan view.

For the purpose of illustration, the iso-potential surfaces 40 are shownas planar surfaces or planes. Actually, the iso-potential surfacestypically will take the form of more complex, closed curvilinearsurfaces, which are orthogonal to the probe axis 34 near the probe, butwhich deviate significantly from planar with increasing distance fromthe probe. The depiction of the surfaces 40 in the drawings aids in theunderstanding of the invention, as coordinate locations in andintersections of the more complex iso-potential surfaces 40 cangenerally be treated equivalent to coordinate locations andintersections of planar surfaces.

As FIG. 2 shows, the differential comparison along the iso-potentialsurfaces 40(1) to 40(5) derives either an in-phase relationship or anout-of-phase relationship between the voltage sensed by the element 16(W_(S))and the voltage at the plane of the sensing electrode (W_(O)),depending upon the location of the sensing element 16 relative to theiso-potential surface 40 of the electrode 26 along which thedifferential measurement is acquired.

More particularly, FIG. 2 shows the sensing element 16 to be located tothe right of iso-potential surfaces 40(1), 40(2), and 40(3) and to theleft of the iso-potential surfaces 40(4) and 40(5). In this orientation,when either surface 40(1) or 40(2) or 40(3) is the surface along whichthe differential measurement is taken, the differential comparison ofW_(S) and W_(O) indicates an out-of-phase relationship between the twowaveforms. The out-of-phase relationship indicates that theiso-potential surfaces 40(1), 40(2), or 40(3) are located in a proximaldirection relative to the sensing element 16, meaning that the sensingelement 16 is located between these iso-potential surfaces and thereturn electrode 28.

Conversely, when the differential measurement is acquired along eithersurface 40(4) or 40(5), the differential comparison of W_(S) and W_(O)indicates an in-phase relationship between the two waveforms. Thein-phase relationship indicates that the iso-potential surfaces 40(4) or40(5) are located in a distal direction relative to the sensing element16, meaning that the these iso-potential surfaces are located betweenthe sensing element 16 and the return electrode 28.

The central processing unit 18 controls the switch unit 72 toelectronically switch the electrodes 26(2) to 26(5) to perform adifferential comparison of the waveform W_(S) of the sensing electrode16 and the waveform W_(O) of the switched-on electrode 26. In FIG. 2,the differential comparison of W_(S) and W_(O) will shift from anout-of-phase condition to an in-phase condition when the measurement isacquired along the iso-potential surface 40(4). The switch point betweenout-of-phase and in-phase conditions marks the longitudinal orientationof the sensing element 16 (and thus the operative element 12) along theaxis 34 of the locating probe 14, i.e., between iso-potential surface40(3) and iso-potential surface 40(4).

The central processing unit 18 can also perform a differentialcomparison between the signal amplitude of the acquired waveform A_(S)and the signal amplitude of the waveform A_(O) at the switched-onsensing electrode 26. From the differential amplitude comparison, thecentral processing unit 18 derives the latitudinal orientation of theoperative element 12 perpendicular to the axis 34 of the locating probe14, i.e., the vertical distance within the space S between the operativeelement 12 and the probe axis 34. The magnitude of the differencebetween A_(S) and A_(O) increases as a function of increasing distancebetween the sensing element 16 and the plane of the switched-onelectrode 26. The function governing the increase of the amplitudedifferential over distance can be empirically determined, or bedetermined by finite element analysis.

There are various electrical configurations, analog or digital, that canbe used to carry out the above differential comparisons. FIG. 3 showsone representative implementation.

In FIG. 3, the system 10 includes an address bus 64, which couples thecentral processing unit 18 to the first-described switch unit 30. Thefirst switch unit 30 is also coupled to a transmitting electrode, e.g.electrode 26(1), and return electrode 28. The central processing unit 18conditions the first switch unit 30 via the bus 64 to distribute thealternating current output of the oscillator 22 in a prescribed fashionin oil parallel to at least the electrodes 26 (1) for return through thereturn electrode 28.

In this arrangement, the system 10 also includes a data acquisitionsystem (DAQ) 68. The DAQ 68 includes a differential amplifier 70. Thesensing element 16 is coupled to the non-inverting (+) input of theamplifier 70.

The DAQ 68 further includes the second electronic switch unit 72, whichis independently coupled to the electrodes 26(1) to 26(5). The centralprocessing unit 18 conditions the second switch unit 72 via a secondaddress bus 74 to couple a selected one transmitting electrode 26 on thelocating probe 14 to the inverting (-) input of the amplifier 70.

In this arrangement, the differential amplifier 70 reads the electricalpotential of the sensing element 16 with respect to that of theswitched-on transmitting electrode 26, then coupled to the amplifier 70by the switch unit 72. The output 71 of the amplifier 70 is an ACvoltage signal.

The DAQ 68 also includes a synchronized rectifier 76 and peak detector78. The rectifier 76 receives the AC signal voltage output of theamplifier 70 and acquires its phase relative to the phase at the outputof the oscillator 22. The detector 78 determines the peak amplitude ofthe AC voltage signal output 71 of the amplifier 70. In an alternativeimplementation, the rectifier 76 and detector 78 can take the form of asynchronized phase detector, or any other element that detects phase andamplitude (whether as an RMS value, peak value, average rectified value,or otherwise).

The output of the detector 78 is an analog signal having a valuecorresponding to the peak amplitude of the AC output of the amplifier70, and a sign (+ or -) denoting whether the AC voltage output is inphase with the oscillator 22 (+) or out of phase with the oscillator 22(-).

The DAQ 68 registers this analog signal in association with theswitched-on electrode 26 then-coupled to the amplifier 70 in a sampleand hold element 80. An analog to digital converter 82 converts theanalog signals to digital signals for processing by the centralprocessing unit 18. A suitable control bus 54 couples the sample andhold element 80, converter 82, and differential amplifier 70 to thecentral processing unit 18 for coordination and control functions. Forexample, the central processing unit 18 can set the sampling rate of thesample and hold element 80, the input range of the converter 82, and theamplification of the amplifier 70.

In determining the longitudinal location of the sensing element 16, thecentral processing unit 18 conditions the first switch unit 30 toconnect the return electrode 28 to the isolated ground 38 of theoscillator 22.

The central processing unit 18 also conditions the first switch element30 to direct AC current flow from the oscillator 22 in parallel to themost proximal transmitting electrode 26(1), while also conditioning thesecond switch unit 72 to couple the switched-on transmitting electrode26(1)to the inverting input of the differential amplifier 70. Theamplifier 70 subtracts the electrical potential measured at theswitched-on electrode 26(1) from the electrical potential measured bythe sensing element 16. The differential potential times the gain of theamplifier 70 constitutes the input to the rectifier 76.

The rectifier 76 senses the synchronization of the phase of its inputvoltage relative to the phase of the oscillator 22, while the detector78 senses the peak voltage. This signed analog value is passed throughthe sample and hold element 80, converted to a digital format by theconverter 82 and registered by the central processing unit 18 inassociation with the identity of the switched-on transmitting electrode26(1).

The central processing unit 18 next conditions the second switch unit 72to couple the electrode 26(2) to the inverting input of the differentialamplifier 70. The central processing unit 18 processes the signalobtained for the switched-on electrode 26(2) in the same fashion as theoutput voltage signal for the first switched-on electrode 26(1). Thecentral processing unit 18 proceeds in like fashion sequentially throughall the remaining electrodes 26 (3), 26(4), and 26(5), deriving andprocessing the output voltage signal for each switched-on electrode 26.The processor 18 registers the digitally converted peak voltages andphase synchronization for each switched-on transmitting electrode 26(1)to 26(5).

Typically, it can be expected that the electrical capacitances andinductances of tissue in and about the space S are minimal. Therefore,the synchronization of the phase of the output voltage signal of theamplifier 70 relative to the phase of the oscillator 22 will varydepending upon whether the sensing element 16 is located to the left orto the right of the transmitting electrode 26 then-coupled to theinverting input of the amplifier 70 (as FIG. 2 shows).

If the switched-on electrode 26 is located to the left of the sensingelement 16 (as FIG. 2 shows for electrodes 26(1), 26(2), and 26(3)), theoutput voltage signal of the amplifier 70 will be out of phase withrespect to the phase of the oscillator 22 (i.e., that analog signalreceived by the sample and hold element 80 will have a (-) sign). Thisis because the potential of the sensing element 16 acquired at thenoninverting input of the amplifier 70 (during the positive phase ofoscillator output) will be more negative than the potential acquired atthe electrodes 26(1), 26(2), and 26(3), which are sensed at theinverting input of the amplifier 70. A_(S) long as the potential of thesensing element 16 remains more negative under these conditions, theoutput voltage signal of the amplifier 70 remains negative, indicatingan out of phase condition.

If the switched-on electrode 26 is located to the right of the sensingelement 16, (as FIG. 2 shows for transmitting electrode 26(4) and26(5)), the output voltage signal of the amplifier 70 will be in phasewith respect to the phase of the oscillator 22. This is because thepotential of the sensing element 16 acquired at the noninverting inputof the amplifier 70 (during the positive phase of oscillator output)will be more positive than the potential at the electrodes 26(4) and26(5) sensed at the inverting input of the amplifier 70. A_(S) long asthe potential of the sensing element 16 remains more positive underthese conditions, the output voltage signal of the amplifier 70 remainspositive, indicating an in phase condition.

The central processing unit 18 monitors the output of the peak detector78 to determine where the output changes sign, by turning from (-) to(+) or vice versa. In FIG. 2, this transition occurs between switched-onelectrode 26(3) and switched-on electrode 26(4). The iso-potentialsurface 40(3) associated with the electrode 26(3) sets the longitudinalcoordinate of the sensing element 16, and thus the operative element 12.

To determine the latitudinal coordinate of the sensing element 16 usingdifferential amplitude sensing, the central processing unit 18conditions the first switch unit 30 to direct AC current flow from theoscillator 22 to the particular switched-on electrode 26(3) at which thephase transition occurred. The central processing unit 18 conditions thesecond switch unit 72 to couple the particular phase transitionelectrode 26(3) to the inverting input of the differential amplifier 70while sensing element 16 is coupled to the noninverting input of theamplifier 70. The amplifier subtracts the electrical potential measuredat the phase-transition electrode 26(3) from the electrical potentialmeasured at the sensing element 16. The differential potential times thegain of the amplifier 70 constitutes the input to the rectifier 76.

The detector 78 senses the peak voltage amplitude of the signal. Theoutput of the peak detector 78 is passed through the sample and holdelement 80 and converted to digital format by the converter 82. Thisdigitally converted peak voltage amplitude is registered by the centralprocessing unit 18. The central processing unit 18 compares the peakvoltage amplitude to a voltage amplitude variation table stored inmemory, which lists variations in peak voltage amplitude as a functionof distance from the plane of the transmitting electrode. The voltageamplitude variation table can be empirically determined or based uponfinite element analysis, taking into account the physical and electricalparameters of the space S.

In a preferred embodiment, a predetermined threshold amplitude isestablished, which corresponds to a nominal distance from thetransmitting electrode, which differentiates between a "close condition"(i.e., equal to or less than the nominal distance) and a "far condition"(i.e., greater than the nominal distance). When the sensed peak voltageamplitude is equal to or less than the threshold amplitude, the centralprocessing unit 18 generates an output that notifies the physician ofthe "close condition" between the sensing element 16 and the switched-ontransmitting electrode 26. When the sensed peak voltage amplitude isless than the threshold amplitude, the central processing-unit 18generates an output that notifies the physician of the "far condition"between the sensing element 16 and the switched-on transmittingelectrode 26. In this way, the physician has at least a qualitativeindication of the position of the sensing element 16 relative to theswitched-on transmitting electrode 26. In one embodiment, the physiciancan indicate through input to the central processing unit 18 themagnitude of the nominal distance, or, alternatively, establish a rangeof distances that progressively indicate a "closest", "closer" and"close" variation of positions.

In another embodiment, the sensing of the voltage amplitude isaccomplished in a way that also provides information regarding theorientation of the sensing element 16 relative to the switched-ontransmitting electrode 26. More particularly, as shown in FIG. 1, theoperative element 12 can carry a second sensing element 16' spaced aknown distance apart from the first mentioned sensing element 16. Inthis arrangement, one or more transmitting electrodes on one probe areswitched on in sequence or simultaneously to transmit the energy fieldto an indifferent patch electrode, which serves as a return path.Sensing individually at each sensing element 16 and 16' provides, notonly a peak voltage amplitude, but also, through a comparison ofrelative phases and amplitudes at each element 16 and 16', informationregarding the orientation of the operative element 12 itself. Forexample, the central processing unit 18 can differentially compare theamplitude at sensing element 16' with the amplitude at sensing element16 to determine that element 16 is further away from the transmittingelectrodes than element 16'. This indicates that the orientation of theoperative element 12 is skewed within the space S.

In an alternative embodiment, the second sensing element 16' cancomprise the return path for the transmitting electrode 26, instead of areturn path electrode 28 carried by the locating probe 14. In yetanother alternative embodiment, the energy field can be transmitted byone of the elements 16 or 16' and returned by the other one of theelement 16' or 16. In either of theses arrangements, the peak voltageamplitude is sensed by an electrode on one of the locating probes.

B. Multiple Locating Probes

FIGS. 4 and 5 show a system 100 that locates an operative element 102within a space (designated S) by generating an energy waveform field 110using two locating probes 106 and 108. Each locating probe 106 and 108is generally like the locating probe 14 shown in FIGS. 1 and 2, havingat least one transmitting electrode and at least one return electrode.For purpose of illustration, the locating probes 106 and 108 each carrymore electrodes than the probe 14. The electrodes carried by thelocating probe 106 are designated X(1) to X(6) and the electrodescarried by the locating probe 108 are designated Y(1) to Y(5). Eachlocating probe 106 and 108 also includes a return electrode, designatedRX for probe 106 and RY for probe 108.

The locating probes 106 and 108 are positioned relative to each other inor near the space, such that their elongated axes, respectively 120 and122, are not parallel, but extend at an angle. In the illustratedembodiment, the angle is about 90°, but other smaller or larger anglescan be used. Furthermore, the locating probes 106 and 108 need not liein the same plane.

A_(S) in the FIGS. 1 and 2 embodiment, the operating element 102 carriesa sensing element 104.

Like the system 10 described in FIGS. 1 and 2, the operation of thesystem 100 is governed by a central processing unit 112. The centralprocessing unit 112 connects the waveform output of an oscillator 114through a switch unit 116 between the selected transmitting electrodeY(1) and X(1) on the locating probes 106 and 108 and the respectivereturn electrode RY and RX, which is also couple to isolated ground orpatient ground 118. The central processing unit 112 also couples thesensing element 104 to the electrodes of the probes 106 and 108 (via theswitch unit 117 and DAQ 119) along the iso-potential surfaces TX(1) toTX(6) and TY(1) to TY(5) in the energy waveform field 110. Due to theangular placement of the locating probes 106 and 108, the iso-potentialsurfaces TX(1) to TX(6) of the probe 106 intersect the iso-potentialsurfaces TY(1) to TY(5) of the probe 108. FIG. 4 shows the intersectingiso-potential surfaces TX and TY in side view. FIG. 5 shows theintersecting iso-potential surfaces TX and TY in perspective view.

As previously described, the central processing unit 112 performs adifferential comparison of the waveform W_(S) to the waveform outputW_(O) when each of the transmitting electrodes X(1) to X(6) and Y(1) toY(5) are switched on. The differential comparison derives either anin-phase or relationship an out-of-phase relationship between W_(S) andW_(O), depending upon the location of the sensing element 104 relativeto the iso-potential surface TX(N) or TY(N) of the switched-on voltagesensing electrode X(N) or Y(N).

More particularly, FIG. 4 shows the sensing element 104 to be located tothe right of (or above, in the vertical orientation shown in FIG. 4) theiso-potential surfaces TX(1) to TX(4) and to the left of (or below, fromthe vertical orientation shown in FIG. 4) the iso-potential surfacesTX(5) and TX(6). In this orientation, when either plane TX(1) or TX(2)or TX(3) or TX(4) is switched-on for sensing, the differentialcomparison of W_(S) and W_(O) indicates an out-of-phase relationshipbetween the two waveforms. This means that the sensing element 104 islocated between these planes and the return electrode RX. Conversely,when either plane TX(5) or TX(6) is switched-on for sensing, thedifferential comparison of W_(S) and W_(O) indicates an in-phaserelationship between the two waveforms. This means that these planes arelocated between the sensing electrode 104 and the return electrode RX.

The central processing unit 112 controls the switch unit 116 toelectronically switch the electrodes on, sequentially from most proximalto most distal, i.e., sequentially from left to right (or from bottom totop, in the vertical orientation shown in FIG. 4) from X(1) to X(6).This sequentially switches on differential sensing along theiso-potential surfaces TX(1) to TX(6).

For each switched-on electrode X(1) to X(6), the central processing unit112 performs (via the DAQ 119) a differential comparison of the waveformW_(S) of the sensing electrode 104 and the waveform W_(O) of theswitched-on electrode X(N). In FIG. 4, the differential comparison ofW_(S) and W_(O) will shift from an out-of-phase condition to an in-phasecondition when measurement occurs along the iso-potential surface TX(5).The switch point between out-of-phase and in-phase conditions marks thelongitudinal orientation of the sensing element 104 (and thus theoperative element 102) along the axis 120 of the locating probe 106,i.e., between iso-potential surface TX(4) and iso-potential surfaceTX(5).

The central processing unit 112 can also perform a differentialcomparison between the signal amplitude of the sensed waveform A_(S) andthe signal amplitude of the waveform at the switched-on transmittingelectrode A_(O). From the differential amplitude comparison, the centralprocessing unit 112 derives the latitudinal orientation of the operativeelement 102 perpendicular to the axis 120 of the probe 106, i.e., thevertical distance within the space S between the operative element 102and the probe axis 120.

The same methodology is repeated along the locating probe 108. FIG. 4shows the sensing element 104 to be located to the right of theiso-potential surfaces TY(1) to TY(2) and to the left of theiso-potential surfaces TY(3), TY(4), and TY(5). The central processingunit 112 controls the switch unit 117 to electronically switch on thetransmitting electrodes, sequentially from most proximal to most distal,i.e., sequentially from left to right, Y(1) to Y(5). This sequentiallyswitches on differential sensing along the iso-potential surfaces TY(1)to TY(5).

For each switched-on electrode Y(1) to Y(5), the central processing unit112 performs (via the DAQ 119) a differential comparison of the waveformW_(S) of the sensing element 104 and the waveform W_(O) of theswitched-on transmitting electrode Y(N). In FIG. 4, the differentialcomparison of W_(S) and W_(O) along the probe 108 will shift from anout-of-phase condition to an in-phase condition when iso-potentialsurface TY(3) is switched on. The switch point between out-of-phase andin-phase conditions marks the longitudinal orientation of the sensingelement 104 (and thus the operative element 102) along the axis 122 ofthe locating probe 108, i.e., between iso-potential surface TY(2) andiso-potential surface TY(3).

The central processing unit 112 can also perform a differentialcomparison between the signal amplitude of the sensed waveform A_(S) andthe signal amplitude of the waveform at the switched-on transmittingelectrode A₀ to derive the latitudinal orientation of the operativeelement 102 perpendicular to the axis 122 of the probe 108, i.e., thevertical distance within the space S between the operative element 102and the probe axis 122.

The component parts of the system 100 can incorporate the particularelectrical configuration shown in FIG. 3, or another analog or digitalconfiguration, to carry out the above differential comparisons.

The central processing unit 112 provides a position-indicating output124, which correlates the position of the sensing element 104 (and thusthe operative element 102) within the grid of intersecting iso-potentialsurfaces TX(N) and TY(N). Preferably, the position-indicating output 124is presented to the physician on a display device 126.

The individual identification probes 106 and 108 shown in FIGS. 4 and 5can be assembled into a composite structure 150, as shown in FIG. 6. Inthis arrangement, the structure 150 comprises an, array of flexiblespline elements 152 extending longitudinally between a distal hub 154and a proximal base 156. For purpose of illustration, the structure 150includes four spline elements 152(1) to 152(4) (only 3 spline elementsare visible in FIG. 6). A greater or lesser number of spline elements152 can be present.

Each spline element 152 preferably comprises a flexible body made fromresilient, inert wire or plastic. Elastic memory material such as nickeltitanium (commercially available as NITINOL™ material) can be used.Resilient injection molded plastic or stainless steel can also be used.Each spline element 152 is preferably preformed with a convex bias,creating a normally open three-dimensional basket structure.

The structure 150 is carried at the end of a catheter tube 158. An outersheath 160 slidably advances forward along the catheter tube 158 tocompress and collapses the structure 150 (see FIG. 7) for introductioninto the body region. Rearward movement retracts the slidable sheath 160away from the structure 150, which springs open and assumes itsthree-dimensional shape (as FIG. 6 shows).

In FIG. 6, the geometry of spline elements 152 is both radially andaxially symmetric. Asymmetric structures, either radially or axially orboth, can also be used. Examples of asymmetric arrays of splinestructures are shown in copending U.S. application Ser. No. 08/742,569,filed Oct. 28, 1996 and entitled "Asymmetric Multiple Electrode SupportStructures," which is incorporated herein by reference.

Each spline element 152 carries an array of multiple transmittingelectrodes TE and at least one return electrode RE, as previouslydescribed. Each spline element 152 thus comprises a locating probe. Thestructure 150 comprises an ordered array of multiple location probes,which, in use, create a waveform field 162 about the space bounded bythe spline elements 152.

FIG. 6 shows an operative element 172 movable within the energy waveformfield 162. The operative element 172 carries a sensing element 174.

As before described, a central processing unit 164 sequentially connectsthe waveform output of an oscillator 166 through a switch unit 168 tothe transmitting electrodes TE on each spline element 152 (for example,beginning with the most proximal and moving distally), while couplingthe respective most distal return electrode RE of the spline element 152to isolated ground or patient ground 170. The central processing unit164 also sequentially couples the electrodes TE and the sensingelectrode 174 on the operative element 172 through a switch unit 169 anda DAQ 171 to acquire a differential voltage along a grid of intersectingiso-potential surfaces TP in the energy waveform field 162, in the samemanner shown for the probes 106 and 108 in FIGS. 4 and 5. Thedifferential comparison derives either an in-phase relationship or anout-of-phase relationship between W_(S) and W_(O), depending upon thelocation of the sensing element 174 relative to the transmittingelectrodes along each elongated spline element 152.

The central processing unit 164 can also perform a differentialcomparison between the signal amplitude of the sensed waveform A_(S) andthe signal amplitude of the waveform at the switched-on electrode A_(O)where the phase transition occurs, to derive the latitudinal orientationof the sensing element 174 perpendicular to each spline element 152.

II. Differential Voltage Analysis

A. Relative Proximity Derivation

FIG. 8 shows an alternative embodiment of system 300 that locates anoperative element 302 within a space (designated S), using differentialvoltage analysis instead of differential waveform analysis. The systemgenerates an energy waveform field 310 between two locating probes 306and 308. Each locating probe 306 and 308 includes at least onetransmitting electrode, which are designated X(1) to X(6) for probe 106and Y(1) to Y(6) for probe 108. The operative element 302 carries asensing element 304.

In the illustrated embodiment, the locating probes 306 and 308 arepositioned so that their elongated axes, respectively 320 and 322, arenot parallel, but extend at some angle. In the illustrated embodiment,the angle is about 90°, but other smaller or larger angles can be used.Alternatively, because differential voltage analysis is employed, thelocating probes 306 and 308 in this embodiment can be located in aparallel, mutually facing relationship.

The operation of the system 300 is governed by a central processing unit312. The central processing unit 312 connects the waveform output of anoscillator 314 through a first switch unit 316 to transmit the waveformfrom all transmitting electrodes on one probe 306 to all the electrodeson the other probe 308, which are coupled to the isolated patient ground318. For this reason, the probe 306 will be called the "transmittingprobe" and the probe 308 will be called the "receiving probe." Thereceiving and transmitting functions of the probes 306 and 398 can bereversed. The generated waveform field 310 extends between thetransmitting probe 306 and the receiving probe 308. The waveform can begenerated simultaneously between all electrodes or sequentially alongthe axis of the probes 306 and 308.

As FIG. 8 shows, the waveform field 310 includes iso-potential surfacesT(1) to T(6), which extend between the transmitting-receiving electrodepairs X(1)-Y(1) to X(6)-Y(6).

The central processing unit 312 conditions a second switch element 330to couple each switched-on electrode on the transmitting probe 306 insuccession to inverting (-) input of a differential amplifier 332, whilecoupling the sensing element 304 to the noninverting (+) input. Theamplifier subtracts the electrical potential measured by the electrodecoupled to the inverting input from the electrical potential measured bythe sensing element 304. The differential potential times the gain ofthe amplifier 332 constitutes the input to a rectifier 334.

A detector 336 senses the peak voltage, and the rectifier 334 senses thesynchronization of the phase of the voltage signal relative to the phaseof the oscillator 314. The central processing unit 312 registers thepeak voltage and the synchronization in association.

The synchronization of the phase of the output voltage signal of theamplifier 332 relative to the phase of the oscillator 314 will varydepending upon the location of the most immediately distal iso-potentialsurface to the sensing electrode 304.

More particularly, the output voltage signal of the amplifier 332 willbe in-phase with respect to the phase of the oscillator 314 only whenthe differential amplitude is measured along the iso-potential surfacewhich is most immediately distal to the sensing electrode 304. In FIG.8, the most immediate distal iso-potential surface to the sensingelectrode 304 is T(6), which lies between electrode pairs X(6)-Y(6). Theoutput voltage signal of the amplifier 332 will be out-of-phase withrespect to the phase of the oscillator 314 for the differentialamplitudes measured along the most immediately proximal iso-potentialsurface to the sensing electrode 304, and along all other more proximaliso-potential surfaces. In FIG. 8, the most immediate proximaliso-potential surface is T(5), which lies between electrode pairsX(5)-Y(5) and the remaining more proximal surfaces T(4) to T(1) liebetween electrode pairs X(4)-Y(4) to X(1)-Y(1).

By way of another example, assuming another position of the sensingelement 304' (shown in phantom lines in FIG. 8), the output voltagesignal of the amplifier 332 will be in-phase with respect to the phaseof the oscillator 314 only when the differential amplitude is measuredalong the iso-potential surface T(4), which is the most immediatelydistal to the sensing electrode 304'. The output voltage signal of theamplifier 332 will be out-of-phase with respect to the phase of theoscillator 314 for the differential amplitudes measured along the mostimmediate proximal iso-potential surface T(3) and all other moreproximal iso-potential surfaces T(2) and T(1).

Differential voltage analysis can also be used in association with thecomposite probe structure 150 shown in FIG. 6 or any of the structuresshown earlier.

III. Three-Dimensional Navigation Systems

A. Establishing a Three-Dimensional Navigation System (Using a WaveformDifferential Analysis)

FIG. 9 shows a representative implementation of a three-dimensionalnavigation system 200, which includes three locating probes 204, 206,and 208 positioned within a space S. In the illustrated embodiment, thespace S comprises the interior of a heart. In use, the system 200locates and guides an operative element 202 within the heart. Theoperative element 202 can serve to sense electrical activity in theheart to locate potential ablation sites, or to transmit energy to paceheart tissue, measure impedance, or to ablate. Alternatively, theoperative element 202 can include an imaging element to image tissue,anatomic structures, or lesions formed within the heart. Also, theoperative element can include a cannula to penetrate heart tissue forthe purpose of injecting an ablation media, or to inject a drug or genetherapy agent.

For purpose of illustration, the three locating probes 204, 206, and 208are purposely situated within the heart to provide spaced-apartnavigational points for locating the operative element 202. Furthermore,the probes 204, 206, and 208 are located at different coordinate planes,to create a three-dimensional navigational grid and make triangulationpossible.

In the illustrated embodiment, the probes 204, 206, and 208 areindividually placed at or near known anatomic regions of the heartusing, for example, fluoroscopy or another imaging technology, such asultrasound. This is because potential ablation sites within the atriaare typically identified by reference to an anatomic landmark within theheart.

It should be appreciated that a single locating probe or multiplelocating probes may be positioned essentially in any region within theheart or in any tissue or vascular region surrounding the heart forpurposes of establishing navigational points of reference to locate theoperative element 202. Any region of placement with the body that can beimaged by fluoroscopic or other imaging technology can be selected as apotential navigational site. The region of placement therefore does nothave to represent a particular fixed anatomic site. For example,establishing a three-dimensional navigation system for use within agiven heart chamber, one or more locating probes can be located withinthe heart chamber, another one or more probes may be located in adifferent chamber, and yet another one or more locating probes can belocated at an epicardial location outside the interior of the heart.

In the illustrated embodiment, the first locating probe 204 ispositioned in region of the high right atrium; the-second locating probe206 is positioned in the region of the right ventricular apex; and thethird locating probe 208 is positioned in the region of the coronarysinus. The three probes 204, 206, and 208 are located on differentcoordinate planes, so that the probe axes extend in mutually nonparallelrelationships.

Each locating probe 204, 206, and 208 includes multiple transmittingelectrodes TE and a distal return electrode TR, which function in themanner previously described and shown in FIG. 1. A transmittingelectrode TE and the return electrode TR on each probe 204, 206, and 208are coupled via electronic switch units 210 to an oscillator 212 tocreate an energy waveform field 216.

The operative element 202 carries a sensing element 218, which can alsocan serve as an ablation electrode or as sensing electrode. The sensingelement 218 is coupled to the central processing unit 214 in the mannerpreviously described to sense the waveform quantity W_(S) within thefield 216.

A DAQ 68 acquires differential waveforms along multiple iso-potentialsurfaces TP, one associated with each electrode TE on each probe 204,206, and 208. A_(S) shown in FIG. 9, because the probes 204, 206, and208 are located-at different coordinate planes, the multipleiso-potential surfaces TP form intersection points within the field 216.

The central processing unit 214 employs the DAQ 68 previously described(see FIG. 3) to differentially compare W_(S) to W_(O) for eachswitched-on electrode TE and locate regions of phase transitionsrelative to each probe 204, 206, and 208. In addition, the centralprocessing unit 214 can also perform a differential comparison betweenthe signal amplitude of the sensed waveform A_(S) and the signalamplitude of the waveform at the switched-on transmitting electrodeA_(O) where the phase transition occurs to derive the latitudinalorientation of the sensing element 218 perpendicular to the axis of eachprobe 204, 206, 208.

The central processing unit 214 generates a position-indicating output220, which locates the sensing element 218 (and thus the operativeelement 202 itself) within the matrix of intersecting iso-potentialsurfaces TP generated by the three probes 204, 206, and 208.

B. Establishing a Three-Dimensional Navigation system (Using anIterative Voltage Analysis)

FIG. 17 shows a three dimensional system 500, which conducts aniterative differential voltage analysis to determine the location of anoperative element 502 within a space S peripherally bounded by multiplelocating probes 504. In FIG. 17, the multiple locating probes 504 areassembled together by a distal hub 506 and a proximal base 508 into acomposite, three-dimensional basket structure 510 of the type previouslyshown and described in FIG. 6. However, it should be appreciated thatthe multiple locating probes 504 need not be assembled together in acomposite structure, but exist as separate probes located about thespace S, in the manner shown in FIG. 9, as previously described.

The composite structure 510, however, is well suited for use within theheart and can perform other functions in addition to navigation. Forexample, the composite structure 510 can serve to transmit electricalsignals to pace heart tissue or to characterize the electricalcharacteristics of the tissue by acquiring tissue impedancemeasurements. The composite structure can also serve to sense electricalactivity in myocardial tissue to acquire electrograms for heart mappingprocedures.

The composite structure 510 shown in FIG. 17 includes eight locatingprobes 504, and each probe, in turn, carries eight electrodes 505, for atotal of sixty-four electrodes 505 positioned about the space S. FIG. 17identifies the electrodes 505 by the designation (A,B), where A=1 to pand B=1 to e, where p is the total number of probes 504 and e is thenumber of electrodes 505 on each probe 504 (in the illustratedembodiment, p=8 and e=8).

The system 500 includes a central processing unit 512, which couples avoltage source 514 to a transmitting electrode 516 carried by theoperative element 502. In FIG. 17, an indifferent electrode 518, carriedas a patch on the exterior of the patient, comprises the voltage return,which is, in turn, coupled to isolated or patient ground 520.Alternatively, another electrode carried by the operative element 502can serve as the voltage return. The electrode 516 creates a voltagefield 517 within the space S, which varies in detected amplitude at eachprobe electrode 505 according to its distance from the transmittingelectrode 516.

The system 500 includes a data acquisition element 522 coupled to thecentral processing unit 512 and to a switch element 524. The switchelement 524 individually conditions each electrode (A,B) to sensevoltage existing at its location within the field 517, which the dataacquisition element 522 samples and holds, in the manner previouslydescribed, e.g., see FIG. 3.

The central processing unit 512 includes a processing component 526which derives a position-indicating output 528 based upon the voltagedistribution sensed by the electrodes (A,B) on the probes 504. FIG. 18shows the steps of a preferred algorithm 530 for deriving the output528.

As FIG. 18 shows, the algorithm 530 includes, as a first step 532,establishing an estimated coordinate position P(x, y, z)_(EST) for thetransmitting electrode 516 on the operative element 502 within the spaceS, where x is the x-field coordinate, y is the y-field coordinate, and zis the z-field coordinate.

For example, P (x, y, z)_(EST) can be initially arbitrarily set atP(O,O,O), which is at the geometric center of the voltage field 517(designated as GC in FIG. 17). Alternatively, differential waveformanalysis, or differential voltage analysis, or amplitude analysis, asdescribed above, alone or in combination, can also be used to moreaccurately estimate P (x, y, z)_(EST). By way of another example,position indicating methodologies disclosed in copending patentapplication Ser. No. 08/320,301, filed Oct. 11, 1994 and entitled"Systems and Methods for Guiding Movable Electrode Elements WithinMultiple Electrode Structures" can also be used to provide a moreaccurate initial position estimate P(x, y, z)_(EST). To increaseprocessing efficiencies, multiple signals that are orthogonal from asignal processing standpoint (for example, waveform signals of differentfrequencies, waveform signals of the same frequency but which differ by90° in phase, and waveforms from uncorrelated white noise sources) maybe transmitted simultaneously in the manner shown in FIG. 22 (as will bedescribed in greater detail later).

In the next step 536, the algorithm 530 computes the distance ΔD(A,B)between each probe electrode (A,B) and the transmitting electrode 516 atP(x,y,z)_(EST). The distances ΔD(A,B) can be normalized to facilitateanalysis. The algorithm then applies a preestablished, mathematicalvoltage-to-distance function 534 to derive the estimated voltageV(A,B)EST at each electrode (A,B), based upon ΔD(A,B). In effect, thealgorithm 530 constructs an estimated voltage distribution matrix, whichwould exist, according to the function 534, if P (x, y, z)_(EST) was theactual voltage transmission point. The voltage-to-distance function 534can be empirically determined or be based upon finite element analysisand stored in memory accessible to the central processing unit 512.A_(S) a next step 538, the algorithm 530 derives an estimated orexpected voltage differential V(A,B)_(EST) for each electrode 505.

In the next step 540, the algorithm 530 receives as input V(A, B)_(ACT),where V(A, B)_(ACT) is the measured voltage value acquired by operationof the data acquisition element 522 at each probe electrode (A,B). AsFIG. 19 shows, the algorithm 530, in this step 540, creates a measuredvoltage distribution pattern 560 based upon the values for V (A,B)_(ACT), which plots (on the Y-axis) the sensed voltage values for eachelectrode (numbered 1 to 64 on the X-axis). The algorithm 530 creates anestimated voltage distribution pattern 562 based upon the values for V(A, B)_(EST), which plots (on the Y-axis) the estimated voltage valuesfor each electrode (again numbered 1 to 64 on the X-axis).

As a next step 542, The algorithm 530 matches the voltage distributionpattern 560 with the voltage distribution pattern 562 to derive avoltage matching coefficient VM_(COEF).

The value of the voltage matching coefficient VM_(COEF) for a given P(x,y, z)_(EST) increases as P(x, y, z)_(EST) coincides with the actuallocation of the transmitting electrode 516. That is, the value of thevoltage matching coefficient increases in relation to the proximity ofthe transmitting electrode 516 to the estimated position P(x,y,z)_(EST).

The central processing unit 512 can derive the matching coefficientVM_(COEF) in various conventional ways, for example, by employingpattern matching; matched filtering; or cross correlation. Examples ofusing these techniques to derive matching coefficients appear incopending U.S. patent application Ser. No. 08/390,383, filed Feb. 17,1995 and entitled "Systems and Methods for Examining Heart TissueEmploying Multiple Electrode Structures and Riving Electrodes," which isincorporated herein by reference.

In the next step 544, the algorithm 530 determines whether VM_(COEF) isthe "best", i.e., whether it is maximized under the processing rulesapplied. For the first iteration, and for all subsequent iterations wereVM_(COEF) is not maximized, the algorithm 530 applies (in step 546) apreselected incremental correction factor Δx to the x coordinate, factorΔy to the y coordinate, and factor Δz to the z coordinate of theestimated position of the transmitting electrode 516 to create a newestimated position P(x+Δx, y+Δy, z+Δz)), which become the newcoordinates for an estimated position P(x,y,z)_(EST). The algorithm 530ago then loops through the foregoing steps 536, 538, 540, 542, and 544,to derive an iterated voltage matching coefficient VM_(COEF) based uponthe new estimated location. The algorithm 530 iteratively selects Δx,Δy, and Δz until a best (maximum value) voltage matching coefficientVM_(COEF) is achieved in step 544. The coordinates P(x,y,z)_(EST) at thebest, maximum voltage matching coefficient VM_(COEF) become theposition-indicating output 528, as shown in step 548 in FIG. 18.

There are various ways in which the iteration of the x-, y-, and z-coordinates can be accomplished. For example, the algorithm 530 caniterate the x-coordinate alone (keeping the y- and z-coordinatesconstant) until a best voltage matching coefficient VM_(COEF) isachieved, then fix the x-coordinate at that value and iterate they-coordinate alone (while also keeping the z-coordinate constant) untilanother best voltage matching coefficient VM_(COEF) is achieved, andthen fix the y-coordinate at that value and iterate the z-coordinatealone (keeping the previously fixed x- and y-coordinates constant),until another best voltage matching coefficient VM_(COEF) is achieved.The algorithm 530 then loops back through this process, until the bestvoltage matching coefficient VM_(COEF) is obtained for each local x-,y-, and z-coordinate, as well as for P(x, y, z)_(EST) overall.

Alternatively, the x-, y-, and z-coordinates, can be simultaneouslyincremented to maximize the voltage matching coefficient VM_(COEF) forP(x,y,z)_(EST), using, for example, a conventional maximum gradientmethod.

Due to its iterative nature, the algorithm 530 shown in FIG. 18 correctsfor distortion of the locating probes caused by exposure to dynamicconditions within a body cavity, such as within a beating heart chamber.The iterative nature of the algorithm 530 also corrects for electrical"noise" caused, for example, by the inherent electrical resistance ofthe electrodes and associated electrical wiring.

Furthermore, the iterative differential voltage analysis just describedalso makes possible the generation of an error signal, should theposition of the operative element 502 stray beyond the energy field 517.Should this event occur, the estimated voltage and the actual voltagebecome mirror images. This outcome, when sensed by the centralprocessing unit 512, can command the generation of an out-of-field errorsignal.

In an alternative embodiment, the central processing unit 512 canincorporate a neural network 600 (see FIG. 17), which has been trainedon experimentally acquired sets of voltage distribution data correlatedwith known positions of the transmitting electrode 516. Once thetraining phase is completed, the network 600 can instantaneously outputthe position-indicating output 528, based upon input from the dataacquisition element 522 of voltage distribution data sensed by the probeelectrodes 505 during transmission of voltage by the electrode 516.

C. Displaying Three-Dimensional Navigational Information

As FIG. 9 shows, the position-indicating output 220 (or, in theembodiment shown in FIG. 17, the output 528) is preferably processed forviewing on a display device 221. In a preferred embodiment (see FIG.10), the central processing unit 214 includes an input 222 that receivesinformation pertaining to the position and orientation of the locatingprobes 204, 206, and 208 within the heart. The input 222 also receivesinformation pertaining to the shape and size of each locating probe 204,206, and 208. The central processing unit 214 includes functionalalgorithms 224, which set guidance parameters based upon the inputinformation. These guidance parameters are used by the centralprocessing unit 214 to analyze the spatial variations of the electricwaveform field generated by the locating probes 204, 206, and 208. Theguidance parameters govern the processing of differential comparisondata to create the position-indicating output 220 for display on thedevice 221. The processed position-identifying output aids the physicianin locating and guiding the operative element 202 in real time.

In a preferred embodiment (see FIG. 10), the probes 204, 206, and 208 ofthe system 200 are members of a family 209 of locating probes. Thevarious probes comprising the family 209 are characterized by differentgeometries, different densities of transmitting and return electrodes,and other structural and functional differences. In this embodiment,each probe 204, 206, and 208 within the family 209 includes anidentification component 270. The identification component 270 carriesan assigned identification code XYZ. The code XYZ identifies the shapeand size of the electrode-supporting part of the probe and thedistribution of electrodes carried thereon, in terms of the number ofelectrodes and their spatial arrangement. The structure-specificinformation contained in the code XYZ aids the central processing unit214 in creating a positioning matrix based upon the locating probes whendeployed.

In the illustrated embodiment (see FIG. 10), the coded component 270 islocated within the handle 230 attached to the proximal end of thecatheter tube 232 that carries the locating probe 204, 206, and 208.However, the component 270 could be located elsewhere in relation to thelocating probe.

The coded component 270 is electrically coupled to an externalinterpreter 278 when the probe is coupled to the central processing unit214 for use. The interpreter 278 inputs the code XYZ that the codedcomponent 270 contains. The interpreter 278 electronically compares theinput code XYZ to, for example, a preestablished master table 280 ofcodes contained in memory. The master table 280 lists, for each codeXYZ, the structure-specific information required to create thepositioning matrix to locate and guide the operative element 202 withinthe waveform field 216. The functional algorithms 224 of the centralprocessing unit 214 set location and guidance parameters based upon thecode XYZ.

Because knowledge of the physical characteristic of the locating probeand the spatial relationship of the electrodes it carries is importantin setting accurate location and guidance parameters, the algorithms 224preferably disable the central processing unit 214 in the absence of arecognizable code XYZ. Thus, only probes of the family 209 possessing acoded component 270 carrying the appropriate identification code XYZ canbe used in association with the processing element 214.

The coded component 270 can be variously constructed. It can, forexample, take the form of an integrated circuit 284 (see FIG. 11), whichexpresses in digital form the code XYZ for input in ROM chips, EPROMchips, RAM chips, resistors, capacitors, programmed logic devices(PLD's), or diodes. Examples of catheter identification techniques ofthis type are shown in Jackson et al. U.S. Pat. No. 5,383,874, which isincorporated herein by reference.

Alternatively, the coded component 270 can comprise separate electricalelements 286 (see FIG. 12), each one of which expresses an individualcharacteristic. For example, the electrical elements 286 can compriseresistors (R1 to R4), comprising different resistance values, coupled inparallel. The interpreter 278 measures the resistance value of eachresistor R1 to R4. The resistance value of the first resistor R1expresses in preestablished code, for example, the number of electrodeson the probe. The resistance value of the second resistor R2 expressesin preestablished code, for example, the distribution of electrodes onthe probe. The resistance value of the third resistor R3 expresses inpreestablished code, for example, the size of the probe. The resistancevalue of the fourth resistor R4 expresses in preestablished code, forexample, the shape of the probe.

It should be appreciated that the three-dimensional basket structure 510shown in FIG. 17 can also carry an identification component 270 havingan assigned identification code XYZ to identify the shape and size ofthe multiple probe structure 510 and the distribution of-electrodescarried thereon. In this arrangement, the structure-specific informationcontained in the code XYZ aids the position derivation component 528 andalgorithm 530 in FIG. 18 to construct the estimated voltage distributionmatrix and analyze sensed voltage differentials.

The central processing unit 512 can also include a component 550 (seeFIG. 17), which electronically determines structure-specific informationto construct the estimated voltage distribution matrix and analyzesensed voltage differentials. In this arrangement, the component 550commands, in sequence, the transmission of voltage from the source 514through a switch unit 554 from each probe electrode (A,B) to theindifferent electrode 518, while sensing voltage with the remainingprobe electrodes through-the switch 524 and data acquisition element522. The component 550 thereby acquires a first set of data from whichthe voltage differential between every electrode (A,B) can be obtained.

The component 550 includes an input 552, through which the component 550acquires data relating to the linear distance between adjacentelectrodes on each probe 504. Typically, the electrodes 505 on eachprobe 504 will be spaced apart by manufacturer at the same lineardistance, so that will typically be only a single linear distance toinput. The physician can manually enter the linear distance informationthrough the input 522. Alternatively, the input 552 of linear distanceinformation can be carried by a coded component 270 as earlier describedas shown in FIG. 10, which is inputted automatically upon coupling theprobe structure 510 to the central processing unit 512. In thisarrangement, more complex linear distance information can be readilyinputted. The linear distance information comprises a second set ofdata.

Knowing the linear distance information between adjacent electrode 505contained in the second set of data, and the sensed voltagedifferentials between these electrodes 505 contained in the second setof data, the component 520 then derives using conventional estimatingtechniques the distances between other, nonadjacent electrodes 505, bothalong a probe 504 and between probes 504. The component 550 generates ageometric output 556, which, like the code XYZ, the output 556identifies the shape and size of the multiple probe structure 510 andthe distribution of electrodes 505 carried thereon.

The output 556 also provides the basis for calculating the interiorvolume of the structure 510. In the heart, the interior volume of thestructure 510 typically will conform to the interior volume of the heartchamber it occupies. The interior volume will also typically dynamicallyadjust to the changing heart chamber volumes during systole anddiastole. The component 550 therefore makes possible the electricalanalysis, for therapeutic or diagnostic purposes, of heart chambervolumes and the changes in heart chamber volume during systole anddiastole. The component 550 can thereby be used, independent of or inassociation with a navigation function, to characterize heart morphologyand function.

In another embodiment, the component 550 can a incorporate a neuralnetwork 558 (see FIG. 17) to generate in situ the distance-to-voltagefunction 534 particular to a given structure 510, based upon theelectrically sensed geometry and distribution of electrodes on thestructure 510. The neural network 558 is first trained on a known set ofdata that have been previously acquired experimentally. For example,using a back-propagation model, the network 558 can be trained topredict a voltage-to-distance function 534 based upon structure-specificinformation. Once the training phase is completed, the network 558 canbe used to predict the voltage-to-distance function in situ.

Based upon information received by the input 222, the central processingunit 214 (or 512 in FIG. 17) electronically constructs athree-dimensional coordinate system representing a virtual image 290 ofthe energy field. 216 (or 217 in FIG. 17) and surrounding tissue mass T.FIG. 13 shows a representative virtual image 290 based upon two locatingprobes. In FIG. 13, the virtual image 290 indicates the position of thelocating probes (designated "Probe X" and "Probe Y" in FIG. 13), as wellshows the geometry and location of the iso-potential surfaces(designated "X(1) to X(3)" and "Y(1) to Y(3)" in FIG. 13). The virtualimage 290 shows the position of the operative element 202 (designated"Device" in FIG. 10) within the energy field 216, as well as displaysthe coordinates of the operative element (designated "Coordinates: X(2)Y(2)" in FIG. 10). The central processing unit 214 continuously performsthe differential comparisons and updates the virtual image 290 toprovide a real time display for viewing by the physician.

IV. Using Multiple Waveforms

The locating and navigation systems of the type previously describedcreate an energy field by applying a single waveform. Multiple waveformscan be simultaneously applied to gain processing efficiencies, providedthe different waveforms are orthogonal from a signal processingstandpoint. Examples of different, orthogonal processing signalsincludes waveform signals of different frequencies, waveform signals ofthe same frequency but which differ by 902 in phase, and waveforms fromuncorrelated white noise sources.

A. Differential Waveform Analysis Using Different Waveforms

FIG. 20 shows a system 700 that locates an operative element 702 withina space S by generating different waveforms using two probes 706 and708.

In many respects, the system 700 shares common elements with the system100 shown in FIG. 4. The locating probe 706 and 708 are generally likethe locating probes 106 and 108 shown in FIG. 4. The electrodes carriedby the locating probe 706 are designated X(1) to X(6) and the electrodescarried by the locating probe 708 are designated Y(1) to Y(5). Eachlocating probe 706 and 708 also includes a return electrode, designatedRX for probe 706 and RY for probe 708. As in FIG. 4, the locating probes706 and 708 are positioned relative to each other in a non-parallelrelationship. As in the FIG. 4, the operating element 702 carries asensing element 704.

The system 700 includes a first waveform source WF1, which is coupled tothe probe 708. The system also includes a second waveform source WF2.The first waveform WF1 is different than but orthogonal to the secondwaveform WF2. In the illustrated embodiment, the waveforms WF1 and WF2have different frequencies, and the sources comprise separateoscillators 720 and 722.

The probe 708 is coupled via a switching unit 710 and a first filter F1for the waveform WF1 to the inverting (-) input of a differentialamplifier 712. The probe 706 is also coupled by a second switching unit714 and a second filter F2 for the WF2 is also coupled to the inverting(-) input of the differential amplifier 712. The sensing element 704carried by the operative element 702 is coupled to the noninverting (+)input of the differential amplifier 712. The output of the differentialamplifier 712 is coupled to a data acquisition element 716. The dataacquisition element 716 includes a rectifier, peak detector, sample andhold element, and analog-to-digital converter coupled as shown in FIG. 3to process the differential output in the manner previously described,under the control of a central processing unit 718.

Under the control of the central processing unit 718, the multipleoscillators 720 to 722 simultaneous apply the waveform WF1 to theelectrode Y(1), for return through the return electrode RY, and thedifferent waveform WF2 to the electrode X(1), for return through thereturn electrode RX.

The central processing unit 718 operates the switch units 710 and 714 tosimultaneously acquire two differential voltages, one for waveform WF1between the sensing element 704 and the electrode Y(1) and the other forwaveform WF2 between the sensing element 704 and the electrode X(1). Thedifferential amplifier 712 thus acquires phase information for twowaveforms simultaneously along iso-potential surfaces TX(1) and TY(1).

In like fashion, the central processing unit 718 operates the switchunits 710 and 714 to simultaneously acquire two differential voltagesfor the waveforms WF1 and WF2 between the sensing element 704 and theelectrodes Y(2)/X(2), then Y(3)/X(3), and so on. In this way, thedifferential amplifier 712 acquires phase information for two waveformssimultaneously along iso-potential surfaces TX(2)/TY(2), thenTX(3)/TY(3), etc. This simultaneously acquired phase information fromtwo waveforms WF1 and WF2 is processed by the data acquisition element716 to provide a position-indicating output. Greater processingefficiencies can therefore be obtained.

B. Signal Amplitude Analysis Using Different Waveforms

FIG. 21 shows a system 800 in which multiple oscillators 802, 804, 806,and 808 apply different waveforms WF1, WF2, WF3, and WF4 simultaneouslyto multiple electrodes, respectively E(1), E(2), E(3), and E(4), of asingle probe 810, through an indifferent return electrode 830. A_(S)above described, the different waveforms WF1, WF2, WF3, and WF4 areorthogonal in a signal processing sense possessing, for example, theypossess different frequencies. Since the waveforms are appliedsimultaneously to all electrodes E(1) to E(4), no input switching isrequired.

All electrodes E(1) to E(4) of the probe 810 are coupled to an outputswitch 812. The output switch 810 is, in turn, coupled to filters F1,F2, F3, and F4 for the frequencies of, respectively, WF1, WF2, WF3, andWF4. The output of the filters F1, F2, F3, and F4 are coupled to theinverting (-) input of a differential amplifier 814. The sensing element816 carried by an operative element 818 is coupled to the noninverting(+) input of the differential amplifier 814.

The output of the differential amplifier 814 is coupled to a dataacquisition element 820. The data acquisition element 820 includes arectifier, peak detector, sample and hold element, and analog-to-digitalconverter coupled as shown in FIG. 3 to process the differential outputin the manner previously described, under the control of a centralprocessing unit 818.

Under the control of the central processing unit 818, the dataacquisition element 820 simultaneously acquires the differentialamplitude of waveform WF1 between the sensing element 816 and theelectrode E(1), the differential amplitude of waveform WF2 between thesensing element 816 and the electrode E(2), the differential amplitudeof waveform WF3 between the sensing element 816 and the electrode E(3),and the differential amplitude of waveform WF4 between the sensingelement 816 and the electrode E(4). As the magnitude of the differenceincreases as a function of increasing distance between the probeelectrodes and the sensing element 816, the data acquisition element 816is able to simultaneously infer distance with respect to each probeelectrode E(1), E(2), E(3), and E(4).

C. Iterative Voltage Analysis Using Multiple Waveforms

FIG. 22 shows a system 900 for conducting an iterative voltage analysisusing multiple waveforms to determine the location of an operativeelement 902 within a space S peripherally bounded by a composite,three-dimensional basket structure 910, like that shown in FIG. 17.

As in FIG. 17, the composite structure 910 in FIG. 22 includes eightlocating probes 904, and each probe, in turn, carries eight electrodes905, for a total of sixty-four electrodes 905 positioned about the spaceS. As in FIG. 17, FIG. 22 identifies the electrodes 905 by thedesignation (A,B), where A=1 to p and B=1 to e, where p is the totalnumber of probes 904 and e is the number of electrodes 905 on each probe504 (in the illustrated embodiment, p=8 and e=8).

Unlike FIG. 17, the operative element 902 carries two energytransmitting electrodes 912 and 914. Multiple oscillators 916 and 918apply different waveforms WF1 and WF2 simultaneously to the electrodes912 and 914. The different waveforms WF1 and WF2 are orthogonal in asignal processing sense possessing, for example, they possess differentfrequencies. As in FIG. 21, since the waveforms are appliedsimultaneously to both electrodes 912 and 914, no input switching isrequired.

In the manner described with respect to the system 500 shown in FIG. 17,a central processing unit 920 conditions the electrode 912 and theelectrode 914 to simultaneously transmit waveform energy WF1 and WF2 toa patch return electrode 922. Each probe electrode (A,B) is coupled viaa switch 924 to two filters F1 and F2 for the frequencies of thewaveforms, respectively, WF1 and WF2. A data acquisition element 926thereby receives simultaneous inputs from two waveforms WF1 and WF2.

For example, the input for the waveform WF1 could provide a sensedvoltage, for use by the algorithm 530 (shown in FIG. 18) in deriving theposition-indicating output 528. The input for the waveform WF2 couldprovide phase and amplitude information for comparison to the phase andamplitude information of waveform WF1, from which the orientation of theoperative element 12 can be ascertained. By using multiple waveforms,the system 900 also make possible the derivation of both location andorientation out.

As shown in phantom lines in FIG. 22, a second operative element 902'could be present within the space S bounded by the basket structure910-. The second operative element 902' carries at least onetransmitting electrode 912'. Under the control of the central processingunit 920, the electrode 912 of the first operative element 902 transmitsthe first waveform WF1, while the electrode 912' of the second operativeelement 902' transmits the second waveform WF2. A data acquisitionelement 926 thereby receives simultaneous inputs from two waveforms WF1and WF2, via the filters F1 and F2. The input for the waveform WF1 couldprovides a sensed voltage, for use by the algorithm 530 (shown in FIG.18) in deriving the position-indicating output 528 for the firstoperative element 902, while the input for the waveform WF2 provides asensed voltage, for use by the algorithm 530 (shown in FIG. 18) inderiving the position-indicating output 528 for the second operativeelement 902'. Using multiple waveforms, the system 900 is thereby ableto provide locating information for multiple operative elements.

With respect to all embodiments in this Specification, which show a dataacquisition element coupled by a switch unit to multiple probeelectrodes, it should be appreciated that parallel, independent dataacquisition channels, each with its own processing components anddirectly coupled to a single probe electrode, could be substituted.

V. Guiding Multiple Electrode Ablation Arrays

FIG. 14 shows a multiple electrode structure 400 located in the rightatrium of a heart. The structure 400 is flexible and carries a steeringmechanism (not shown), use of which flexes the structure 400 intocurvilinear shapes. The structure 400 carries an array of electrodes402, which transmit radio frequency energy to ablate myocardial tissue.

The electrodes 402 are preferably operated in a uni-polar mode, in whichthe radio frequency ablation energy transmitted by the electrodes 402 isreturned through an indifferent patch electrode 404 externally attachedto the skin of the patient. Alternatively, the electrodes 402 can beoperated in a bi-polar mode, in which ablation energy emitted by one ormore electrodes 402 is returned an adjacent electrode 402 carried in thestructure 400.

The size and spacing of the electrodes 402 are purposely set forcreating continuous, long lesion patterns in tissue, which are capableof treating atrial fibrillation. FIG. 15 shows a representative long,continuous lesion pattern 406 in tissue T. The long continuous lesionpattern 406 is created by additive heating effects between theelectrodes 402. The additive heating effects cause the lesion pattern406 to span adjacent, spaced apart electrodes 402.

Additive heating effects occur either when the spacing between theelectrodes 402 is equal to or less than about 3 times the smallest ofthe diameters of the electrodes 402, or when the spacing between theelectrodes 402 is equal to or less than about 2 times the longest of thelengths of the electrodes 402. When the electrodes 402 are spaced in oneor both of these manners, the simultaneous application of radiofrequency energy by the electrodes 402, in either a bipolar or unipolarmode, creates the elongated continuous lesion pattern 406 typified inFIG. 15.

U.S. patent application Ser. No. 08/566,291, filed Dec. 1, 1995, andentitled "Systems and Methods for Creating Complex Lesion Patterns inBody Tissue" discloses further details regarding systems and methodsthat create complex long lesion patterns in myocardial tissue. Thisapplication is incorporated herein by reference.

When the predetermined spacing requirements set forth above are not met,the additive heating effects do not occur, and a segmented, orinterrupted, lesion pattern 408 is created. FIG. 16 shows arepresentative interrupted lesion pattern 408 in tissue T. Theinterrupted lesion pattern 408 is characterized lesion areas 412separated by gaps 410 of tissue free of lesions.

An interrupted lesion pattern 408 can also occur, even with properspacing between electrodes 402, because of insufficient contact betweenelectrodes 402 and tissue, or due to other localized effects not withinthe immediate control of the physician. After ablation, intracardiacelectrogram analysis or intercardiac imaging of the ablation region, orboth used in tandem, can be used to uncover the existence of anunintended interrupted lesion pattern 408. In this situation, thephysician can deploy an auxiliary ablation electrode 414 (shown in FIG.14), to ablate tissue in the gaps 410 and thereby complete the desiredlesion pattern.

FIG. 14 includes the three-dimensional locating system 200, which waspreviously described and is shown in greater detail in FIG. 9. Under thecontrol of the central processing unit 214 (previously described), thesystem 200 locates and helps the physician guide the multiple electrodestructure 400 within the right atrium, both before and during theablation procedure.

In FIG. 14, the central processing unit 214 includes a component 416,which records the location of each ablation electrode 402 when ablating.The position of each electrode 402 is recorded in the same manner as theposition of the sensing element 218 of FIG. 9 is derived, usingdifferential comparison of waveform phases between each ablationelectrode 402 and the sequentially switched-on transmitting electrodescarried by the locating probes 204, 206, and 208.

When a lesion gap 410 is detected, the system 200 is operated to recallthe recorded ablation electrode coordinates from the component 416. Fromthe ablation electrode coordinates, the coordinates of the gap 410itself can be determined. Knowing the gap coordinates, the system 200can be used to guide the auxiliary ablation electrode 414 into the gap410. This feedback, which is preferably updated continuously in realtime as the physician moves the auxiliary ablation electrode 414, guidesthe physician in locating the ablation electrode 414 at the chosen gapablation site, to thereby complete the desired lesion pattern.

Various features of the invention are set forth in the following claims.

What is claimed is:
 1. A method for locating a movable electrode,comprising:(a) measuring a voltage value distribution pattern betweenthe movable electrode and each of a further plurality of electrodes; (b)generating an estimated voltage value distribution pattern based upon anestimated coordinate position of the movable electrode; (c) generatingan iterative voltage matching coefficient based on a comparison betweenthe measured voltage value distribution and the estimated voltage valuedistribution; (d) repeating steps (a)-(c) until the value of theiterative voltage matching coefficient is equal to a particular value;and (e) generating a position-indicating output based on the estimatedcoordinate position, the position-indicating output indicative of thelocation of the movable electrode.
 2. The method of claim 1, wherein theestimated coordinate location of the movable electrode is initiallyestablished arbitrarily.
 3. The method of claim 1, wherein the estimatedcoordinate location of the movable electrode is initially established byat least one of differential waveform analysis, differential voltageanalysis, and differential voltage analysis.
 4. The method of claim 1,wherein the estimated coordinate location of the movable electrode isestablished by applying a pre-selected incremental correction factor toa previously established estimated coordinate location of the movableelectrode.
 5. The method of claim 1, wherein the estimated coordinatelocation of the movable electrode is in an x-, y-, z- coordinate system.6. The method of claim 1, wherein steps (a)-(c) are performed until theestimated coordinate position of the movable electrode is a bestestimate.
 7. The method of claim 1, wherein the position-indicatingoutput is generated when the estimated coordinate position of themovable electrode is a best estimate.
 8. The method of claim 1, whereinthe voltage matching coefficient increases in relation to a proximity ofthe actual location coordinate of the movable electrode to the estimatedlocation coordinate of the movable electrode.
 9. The method of claim 1,wherein the voltage matching coefficient is generated by employing oneof pattern matching, matched filtering and cross correlation.
 10. Themethod of claim 1, wherein the plurality of electrodes comprises athree-dimensional array of electrodes, and the movable electrode islocated within the array.
 11. A system for locating a movable electrode,comprising:a movable electrode: a plurality of further electrodes; and aprocessing element coupled to the movable electrode and to each of theplurality of electrodes, the processing element configured to generate aposition-indicating output by iteratively comparing a voltage valuedistribution pattern measured between the movable electrode and each ofthe plurality of further electrodes with an estimated voltage valuedistribution pattern, wherein the position-indicating output isindicative of the location of the movable electrode.
 12. The system ofclaim 11, wherein the estimated voltage value distribution pattern isobtained from an estimated location coordinate of the movable electrode,and wherein the processing element is further configured for generatingan iterative voltage matching coefficient based on the voltage valuedistribution pattern comparison and for applying a preselectedincremental correction factor to the estimated location coordinate ofthe movable electrode, until the iterative voltage matching coefficientreaches a particular value.
 13. The system of claim 12, wherein theposition-indicating output is generated from the estimated locationcoordinate when the iterative voltage matching coefficient reaches theparticular value.
 14. The system of claim 11, wherein the plurality ofelectrodes comprises a three-dimensional array of electrodes.
 15. Asystem for locating an operative element, the system comprising:anoperative element carrying a transmitting element, a plurality oflocating probes, each locating probe carrying one or more sensingelements; and a processing element coupled to the transmitting elementand to each of the one or more sensing elements, the processing elementconfigured to generate a position-indicating output by iterativelycomparing a voltage value distribution pattern measured between thetransmitting element and each of the one or more sensing elements withan estimated voltage value distribution pattern, wherein theposition-indicating output is indicative of the location of thetransmitting element.
 16. The system of claim 15, wherein the estimatedvoltage value distribution pattern is obtained from an estimatedlocation coordinate of the transmitting element, and wherein theprocessing element is further configured for generating an iterativevoltage matching coefficient based on the voltage value distributionpattern comparison and applying a pre-selected incremental correctionfactor to the estimated location coordinate of the transmitting element,until the iterative voltage matching coefficient reaches a particularvalue.
 17. The system of claim 16, wherein the position-indicatingoutput is generated from the estimated location coordinate when theiterative voltage matching coefficient reaches the particular value. 18.The system of claim 17, wherein the particular value reached by theiterative voltage matching coefficient is a best value.
 19. The systemof claim 15, wherein the plurality of locating probes are formed into athree-dimensional basket structure.
 20. The system of claim 15, furthercomprising:a voltage source coupled between the transmitting element andthe processing element to condition the transmitting element to generatea voltage field; a switch coupled to the one or more sensing elements tocondition the one or more sensing elements to sense the voltage field;and a data acquisition unit coupled between the switch and theprocessing element to sample and hold the sensed voltage field.